In recent years, reconfigurable optical add/drop multiplexers (ROADMs) have been put into practical use in order to implement a flexible optical network with a large capacity. A ROADM is provided in for example each node of a WDM transmission system. A ROADM can drop an optical signal of a desired wavelength channel from a received WDM optical signal so as to guide it to a client device. In addition, a ROADM can add a data signal received from a client device to a WDM optical signal.
In order to implement the above operations, a ROADM includes a wavelength selective switch. A wavelength selective switch includes for example an array waveguide grating, a micro machine, a liquid crystal element, etc.
Reconfigurable optical add/drop multiplexers are described in for example Japanese Laid-open Patent Publication No. 2012-119925 and Japanese Laid-open Patent Publication No. 2011-109439.
In order to further increase the capacity of optical networks and/or to increase the flexibility of optical networks, methods that use communication resources (wavelength or frequency in this case) more efficiently are discussed. As an example, a multicarrier modulation that multiplexes a plurality of subcarrier optical signals is discussed. As one scheme for multicarrier modulation, for example orthogonal frequency division multiplexing (OFDM) has been used practically. In the descriptions below, an optical signal in which a plurality of subcarrier optical signals are multiplexed may be referred to as a “subcarrier multiplexed optical signal”.
In order to transmit an arbitrary subcarrier optical signal included in a subcarrier multiplexed optical signal, a technique of processing a wavelength with very small granularity may be requested. However, it is difficult to implement a wavelength selective switch having a steep transmission characteristic. In other words, according to the conventional techniques, it is not easy to process separately each subcarrier optical signal included in a subcarrier multiplexed optical signal. Accordingly, it is difficult for the conventional techniques to narrow sufficiently the wavelength spacing (or the frequency spacing) in a channel/subchannel of an optical network. Note that it is assumed in the descriptions below that a subcarrier multiplexed optical signal belongs to a wavelength-division multiplexed optical signal.